An fireplace insert is a factory built metal fireplace that fits into a pre-existing masonry fireplace.

An open gas fireplace has a single walled B-Vent flue through the roof and takes it's combustion air from the room or through an optional outside air duct to the side of the firebox. A direct vent fireplace has a double walled flue, takes its combustion air from outside, has a fixed glass front, and can vent through a wall or a roof.

An Isokern fireplace is a field assembled fire box of factory built modular masonry elements. It can have a masonry or metal chimney for wood burning fireplaces or a metal B-Vent flue (to the roof) for an open gas fireplace (model IBV). It can have an outside air duct to the firebox side wall. The unit can sit on a combustible floor (reinforced for the weight) but the bottom of the firebox will be 4.5 inches higher than the structural subfloor. For a flush hearth the firebox must be recessed into the floor structure and the required clearance must be met at the perimeter of the unit.

I am not sure of the advantage of building a modular masonry firebox for a gas fireplace. It would be virtually indistinguishable from a metal factory-built gas fireplace.

"Zero clearance" means that a factory-built metal or modular masonry wood or gas fireplace can sit directly on a combustible floor structure. The clearances at the sides, top and at the chimney/flue vary with the individual units and that information can only be found in the manufacturers' installation instructions.

To find a metal factory built gas fireplace with an open front just look in a manufacturer's gas fireplace catalog for the B-Vent models rather than the Direct Vent models and also look for single-wall instead of double wall flues.

Thank you! I saw the IBV on the Isokern website but bet there is a more affordable option than that. This will be in a walk out basement with a cement floor. Does this matter with regard to the flush/not flush debate?

Next question. Can the B-Vent and Isokern share a chimney shaft? When we were planning an Isokern stack we had a masonry chimney and I would like to save that (I guess it was just boxed in around the flues?) The Isokerns flues were just side by side.

My builder said he thought the Isokern IBV had a 6" flue but wasn't sure - this would be quite a bit smaller than the standard one.

Wherever the metal or modular masonry factory built gas or wood burning fireplace is located it would have to be recessed into the floor to have a flush firebox and hearth. The depth varies from 4 to 5 inches. It's all a matter of the individual units passing the required UL test and they are apparently not tested directly on concrete slabs (without the factory made base element). Check with Isokern to see if that is an undocumented possibility.

Gas appliances can often share a flue (and get larger) but wood burning fireplaces cannot share chimneys. Whether a gas fireplace can share a flue needs to be reviewed with the local building department and the manufacturer. I've never seen it shown in installation instructions so I am skeptical about it being allowed.

I think the Isokern B Vent is 8" but it is double walled to allow tighter clearances (1" for double walled, 2" for single walled) so I think the outside diameter is 10". Your builder should look at the installation instructions.

The size of vents and chimneys is primarily determined by the size of the combustion chamber opening (otherwise products of combustion would escape into the room) so an open fireplace of any kind is not going to have a small flue or chimney.

If you want a fireplace you need to design the house around the required vent or chimney rather than the other way around. Inflexible vertical elements like chimneys and stairs are the first to be integrated into a plan and structural configuration so this should have been worked out in the preliminary design phase. Of course, a gas vent can be offset considerably more than a chimney but it can increase the cost quite a bit especially if soffits are involved.

" Is there such a thing as a gas fireplace insert that looks like a real masonry fireplace, without the glass doors or glass front?"

Without doors you have an always open flue sucking air out of the house.

Macv - our plans are currently drafted to include the two stacked Isokerns. A draftsmans error made the fireplace 3" wider on one side (where the flue for the downstairs bypassed the upstairs fireplace). The plans went through several revisions like this until I noticed that the fireplace was slightly off in relation to the coffers on the ceiling. The adjustment of the fireplace to the correct position left the bookshelf useable space at 35.5 inches (not including trim on either side).

I was trying to figure out a way to narrow the fireplace stack and thus widen the bookshelves a bit, particularly since we won't be burning logs in the basement.

As far as the sharing the flue, I think I really meant sharing the chase (is this even the right word?) Each seperate Isokern has an attached flue. The two flues travel side by side out the roof. I was wondering if using a narrower b-vent flue but sharing the same chase was possible?

We also have chimney pots on the chimneys so I wonder if it will put one of the pots off location wise.

I'll talk to my builder in more detail tomorrow.

A direct vent gas fireplace with a glass front usually has a double-walled flue that is 6" inside and 8" outside. It is smaller because no room air is drawn into the fireplace.

Couldn't I vent the direct vent outside on the same level (to the north)? Not that I love the vent on the exterior but it would eliminate the flue out and a chimney pot. . .. the glass front does concern me a bit, although we often burn our fireplace (masonry) now with glass doors closed because the youngest child (and now the puppy) freak out about an open fire!

Also not sure why there is the square in the side of the fireplace - looks like the bypass area for the flue upstairs so maybe sloppy draftmanship??? I wouldn't think you would need a bypass for the basement fireplace.